Production of a real power equilibrium of the phase modules of a converter

ABSTRACT

A converter has at least one phase module, an AC voltage terminal and a DC voltage terminal. A phase module branch is formed between each DC voltage terminal and each AC voltage terminal. Each phase module branch has a series circuit containing submodules which each have a capacitor, a power semiconductor, and submodule sensors for detecting energy stored in the capacitor and with a regulation device for regulating the apparatus in dependence on energy values and predetermined desired values. Therefore unbalanced loading of the energy storage units is avoided. The regulation device has a summation unit for summing the energy values while obtaining branch energy actual values and a device for calculating circuit current desired values in dependence on the branch energy actual values. The regulation device compensates for imbalances in the branch energy actual values in dependence on the circuit current desired values.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to an apparatus for conversion of an electriccurrent with at least one phase module which has an AC voltageconnection and at least one DC voltage connection, with a phase modulebranch being formed between each DC voltage connection and each ACvoltage connection and with each phase module branch having a seriescircuit composed of submodules which each have an energy storage unitand at least one power semiconductor, having submodule sensors fordetection of energy stored in the energy storage unit, with energystorage unit energy values being obtained, and having regulation meansfor regulation of the apparatus as a function of the energy storage unitenergy values and predetermined nominal values.

The invention also relates to a method for conversion of a current bymeans of a converter which has at least one phase module with at leastone DC voltage connection and an AC voltage connection, with a phasemodule branch being formed between each DC voltage connection and the ACvoltage connection and having a series circuit composed of submoduleswhich each have an energy storage unit and at least one powersemiconductor.

An apparatus such as this and a method such as this are already known,for example, from the article by A. Lesnicar and R. Marquardt “AnInnovative Modular Multilevel Converter Topology Suitable for a WidePower Range”, which appeared at Powertech 2003. This discloses aconverter, which is intended for connection to an AC voltage network.The converter has a phase module for each phase of the AC voltagenetwork to be connected to it, with each phase module having an ACvoltage connection and two DC voltage connections. Phase module branchesextend between each DC voltage connection and the AC voltage connection,thus providing a so-called six-pulse bridge circuit. The module branchescomprise a series circuit of submodules which each comprise two powersemiconductors which can be turned off and each have freewheeling diodesconnected back-to-back in parallel with them. The power semiconductorswhich can be turned off and the freewheeling diodes are connected inseries, with a capacitor being provided in parallel with said seriescircuit. Said components of the said modules are connected to oneanother such that either the capacitor voltage or the voltage zero isproduced at the two-pole output of each submodule.

The power semiconductors which can be turned off are controlled by meansof so-called pulse-width modulation. The regulation means forcontrolling the power semiconductors have measurement sensors fordetection of currents, with current values being obtained. The currentvalues are supplied to a central control unit which has an inputinterface and an output interface. A modulator, that is to say asoftware routine, is provided between the input interface and the outputinterface. Inter alia, the modulator has a selection unit and apulse-width generator. The pulse-width generator produces the controlsignals for the individual submodules. The power semiconductors whichcan be turned off are switched by the control signals produced by thepulse-width generator from an on position, which allows current to flowvia the power semiconductors which can be turned off, to an offposition, in which a current flow via the power semiconductors which canbe turned off is interrupted. In this case, each submodule has asubmodule sensor for detection of a voltage dropped across thecapacitor.

Further contributions to the control method for a so-called multi-levelconverter topology are known from R. Marquardt, A. Lesnicar, J.Hildinger, “Modulares Stromrichterkonzept für Netzkupplungsanwendung beihohen Spannungen” [Modular converter concept for network coupling use athigh voltages], which appeared at the ETG Symposium in Bad Nauenheim,Germany 2002, from A. Lesnicar, R. Marquardt, “A new modular voltagesource inverter topology”, EPE' 03 Toulouse, France 2003 and from R.Marquardt, A. Lesnicar “New Concept for High Voltage—Modular MultilevelConverter”, PESC 2004 Conference in Aachen, Germany.

German patent application 10 2005 045 090.3, which has not yet beenpublished, discloses a method for controlling a polyphase converter withdistributed energy storage units. The disclosed apparatus likewise has amulti-level converter topology with phase modules which have an ACvoltage connection, which is arranged symmetrically at the center ofeach phase module, and two DC voltage connections. Each phase modulecomprises two phase module branches which extend between the AC voltageconnection and one of the DC voltage connections. Each phase modulebranch in turn comprises a series circuit of submodules, with eachsubmodule comprising power semiconductors which can be turned off andfreewheeling diodes connected back to back in parallel with them.Furthermore, each submodule has a unipolar capacitor. Regulation meansare used to regulate the power semiconductors and are also designed toadjust branch currents which flow between the phase modules. The controlof the branch currents makes it possible, for example, to actively dampcurrent oscillations, and to avoid operating points with relatively lowoutput frequencies. Furthermore, this makes it possible to achieveuniform loading on all the semiconductor switches which can be turnedoff, as well as balancing of highly unbalanced voltages.

The apparatus mentioned initially has the disadvantage that the realpower consumption of a phase module branch does not always correspondprecisely to the losses. This can result in an unbalanced distributionof the energy stored in each phase module branch. The capacitors in thesubmodules are therefore loaded to different levels, resulting inundesirable consequential phenomena.

BRIEF SUMMARY OF THE INVENTION

The object of the invention is therefore to provide an apparatus and amethod of the type mentioned initially which avoid unbalanced loading ofthe energy storage units in the submodules.

The invention achieves this object on the basis of the apparatusmentioned initially in that the regulation means have an addition unitfor addition of the energy storage unit energy values with branch energyactual values being obtained, and have means for calculation ofcirculating-current nominal values Dvb, Dhgl, Dhge as a function of thebranch energy actual values, with the regulation means being designed tocompensate for unbalances in the branch energy actual values as afunction of the circulating-current nominal values Dvb, Dhgl, Dhge.

On the basis of the method mentioned initially, the invention achievesthe object in that the energy which is stored in each energy storageunit is detected, with an energy storage unit energy value beingobtained, all the energy storage unit energy values of a phase modulebranch are added in order to obtain branch energy actual values, andcirculating-current nominal values are determined as a function of thebranch energy actual, values, with circulating currents in the phasemodules being produced as a function of the circulating-current nominalvalues in order to compensate for unbalances.

For the purposes of the invention, the regulation means are designed tocompensate for unbalances relating to the electrical energy stored inthe submodules. To this end, the energy stored in all the energy storageunits is first of all determined for each phase module branch. This isdone by addition of energy storage unit energy values which eachcorrespond to an amount of energy stored in the energy storage unit ofone submodule. The sum of the energy storage unit energy values resultsin branch energy actual values which correspond to a sum of the amountsof energy of all the energy storage units in one phase module branch.For the purposes of the invention, any unbalance is found by comparisonof the branch energy actual values. Finally, a regulation system is usedto produce circulating currents in order to compensate for theunbalance. The circulating-current nominal values Dvb, Dhgl, Dhge areused for this purpose, and are determined as a function of thedifference between the branch energy actual values. Thecirculating-current nominal values are, finally, supplied to theregulation means, which produce the circulating currents required tocompensate for the unbalances, on the basis of the circulating-currentnominal values Dvb, Dhgl, Dhge. This ensures that there is a balancedload on the submodules.

By way of example, an energy storage unit voltage value which isobtained by measurement of the voltage dropped across the energy storageunit is used as the energy storage unit energy value of a submodule. Incontrast to this, the square of the energy storage unit voltage value isused as the energy storage unit energy value. In principle, any valuewhich can be used as a measure for the energy stored in the respectiveenergy storage unit can be used for the purposes of the invention.

For the purposes of the invention, the energy storage unit in asubmodule may also be composed of a plurality of sub-energy storageunits. The energy storage unit energy value is then the sum of thesub-energy storage unit energy values.

The regulation means expediently comprise a regulator at whose input thecirculating-current nominal values Dvb, Dhgl, Dhge are applied and atwhose output circulating-voltage nominal values are tapped off. By wayof example, the regulator is a proportional regulator. The regulationmeans furthermore comprise a current regulation unit, which linearlycombines various voltage nominal values, including thecirculating-voltage nominal values, with one another, that is to saycombines them by addition and subtraction. The result of this linearcombination of voltage nominal values is branch voltage nominal valueswhich are each associated with one phase module branch. The branchvoltage nominal value or values is or are supplied to drive units, whichare likewise associated with one phase module branch.

The apparatus according to the invention advantageously has a positiveand a negative DC voltage connection, with addition means adding thebranch energy actual values of the phase module branches, which areconnected to the positive DC voltage connection, to form a positivebranch sum and adding the branch energy actual values of the phasemodule branches, which are connected to the negative DC voltageconnection, to form a negative branch sum, and has subtraction meanswhich form the difference between the positive and the negative branchsums in order to obtain a vertical circulating-current nominal value Dvbin order to compensate for any vertical unbalance. A vertical unbalanceexists when the phase module branches which are connected to thepositive DC voltage connection have taken up more or less energy thanthe phase module branches which are connected to the negative DC voltageconnection.

Any vertical unbalance can therefore be found by comparison of thebranch energy actual values, with the branch sum of the phase modulebranches which are connected to the positive DC voltage connection beingsubtracted from the branch sum of the phase module branches which areconnected to the negative DC voltage connection. The resultantdifference represents a measure of the vertical unbalance, thus, in thisway, making it possible to derive a nominal value for the regulation inorder to compensate for the vertical unbalance.

According to one expedient further development relating to this, theapparatus according to the invention has means for production of anetwork-frequency positive-phase-sequence system nominal voltage Uvb1,2, 3 as a function of the vertical circulating-current nominal valueDvb, in order to compensate for the vertical unbalance. Thenetwork-frequency positive-phase-sequence system nominal voltage Uvb1,2, 3 relates to the phase angle of the polyphase AC voltage of theconnected network. In a network-frequency positive-phase-sequencesystem, the voltage which is produced rotates on the vector diagram inthe same rotation direction as the vectors of the AC voltage of theconnected network. As described above, the positive-phase-sequencesystem nominal voltage is applied to other voltage nominal values by theregulation means.

In contrast to this, means can be provided for production of anunbalance voltage Uasym as a function of the circulating-current nominalvalues Dvb in order to compensate for the vertical unbalance. Means suchas this for producing an unbalance voltage are, for example, simpleregulators to whose input the circulating-current nominal values areapplied, with the unbalance voltage Uasym being produced at the outputof the regulator. By way of example, the regulator is a simpleproportional regulator.

Means are advantageously provided for verification of a horizontalunbalance in the same sense, with said means producingcirculating-current nominal values Dhgl as a function of the verifiedhorizontal unbalance in the same sense. In addition to a verticalunbalance, horizontal unbalances are also possible, to be precise whenthe branch energy actual values of the phase module branches which areconnected to the positive DC voltage connection are of differentmagnitude. This applies in a corresponding manner to the branch energyactual values of the phase module branches which are connected to thenegative DC voltage connection. A horizontal unbalance in the same senseoccurs when the unbalance between the positive phase module branches isequal to the unbalance between the negative phase module branches. Ahorizontal unbalance in the opposite sense occurs, in contrast, when theunbalance between the positive phase module branches is the inverse ofthe unbalance between the negative phase module branches.

The apparatus according to the invention therefore advantageously hasmeans for verification of a horizontal unbalance in the same sense, withsaid means producing circulating-current nominal values Dhg1 as afunction of the verified horizontal unbalance in the same sense.

According to one expedient further development relating to this, meansare provided for production of circulating-voltage nominal values uhgl,which are respectively associated with a phase module. Thecirculating-voltage nominal values uhgl are applied to other voltagenominal values by the regulation means.

Within the scope of the invention, means are advantageously provided forverification of a horizontal unbalance in the opposite sense, with saidmeans producing circulating-current nominal values Dhge as a function ofthe verified horizontal unbalance in the opposite sense.

According to one expedient further development relating to this, meansare provided for production of a network-frequencynegative-phase-sequence voltage system uhge as a function of theverified horizontal unbalance in the opposite sense. Thenetwork-frequency negative-phase-sequence voltage system isdistinguished by a voltage whose vector rotates in the oppositedirection to the direction of the AC voltage network in the vectormodel.

According to another exemplary embodiment, means are provided forsimultaneous compensation for vertical and horizontal unbalances in theopposite sense.

According to one expedient further development of the method accordingto the invention, the branch energy actual values of all the phasemodules which are connected to a positive DC voltage connection areadded in order to obtain a positive total sum, and the branch energyactual values of all the phase module branches which are connected to anegative DC voltage connection are added to obtain a negative total sum,with the difference between the positive and the negative total sumbeing formed in order to obtain a vertical circulating-current nominalvalue Dvb. This allows any vertical unbalance to be detected, and to bequantified with the aid of the circulating-current nominal value.

A network-frequency positive-phase-sequence system nominal voltage isadvantageously produced on the basis of the vertical circulating-currentnominal value. The amplitude of the DC nominal value in this caseadvantageously includes a periodic function.

In contrast to this an unbalance nominal voltage is produced on thebasis of the vertical circulating-current nominal value Dvb, by means ofa proportional regulator.

By way of example, any horizontal unbalance in the same sense is foundby forming the branch energy actual values of all the phase modulebranches of a phase module in order to obtain phase module energy sumvalues, by forming the mean value of all the phase module energy sumvalues, and by forming differences from said mean value and each phasemodule energy sum value, obtaining horizontal unbalance-current nominalvalues in the same sense.

According to one expedient further development relating to this,circulating-voltage nominal values are formed by means of a regulatorfrom the horizontal unbalance-current nominal values Dhgl in the samesense, and are applied as a nominal voltage by the regulation means toother voltage nominal values.

According to a further refinement of the invention, the branch energyactual values of all the phase module branches of a phase module aresubtracted from one another in order to obtain phase module energydifference values which are associated with a respective phase. The meanvalue of the phase module energy difference values is then calculatedover all the phases, and the difference from said mean value and therespective phase module energy difference value is determined for eachphase, in order to obtain horizontal unbalance-current nominal valuesDhge1, Dhge2, Dhge3 in the opposite sense.

According to one further development relating to this, anetwork-frequency negative-phase-sequence voltage system uhge1, uhge2,uhge3 is determined from the horizontal unbalance-current nominal valuesDhge1, Dhge2, Dhge3 in the opposite sense.

The branch energy actual value of a phase module branch which isconnected to a negative DC voltage connection is advantageouslysubtracted from the branch energy actual value of the phase modulebranch of the same phase module which is connected to the positive DCvoltage connection, with a phase branch module difference beingobtained, with the phase branch module difference being used as theamplitude of a periodic function which oscillates at the networkfrequency and is associated with a phase module, and with the periodicfunctions of the other phase modules each being phase-shifted so as toform a positive-phase-sequence system nominal voltage. Thepositive-phase-sequence system nominal voltage is once again applied toother nominal values of the regulation system.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

Further expedient refinements and advantages are the subject matter ofthe following description of exemplary embodiments of the invention,with reference to the figures of the drawing, in which the samereference symbols refer to components having the same effect, and inwhich:

FIG. 1 shows a schematic illustration of one exemplary embodiment of anapparatus according to the invention,

FIG. 2 shows an illustration of the equivalent circuit of a submodule ofan apparatus as shown in FIG. 1,

FIG. 3 shows a method for finding any vertical unbalance,

FIG. 4 shows the production of a network-frequencypositive-phase-sequence system voltage,

FIG. 5 shows the production of an unbalance voltage,

FIG. 6 shows the verification of any horizontal unbalance in the samesense,

FIG. 7 shows the verification of any vertical unbalance in the oppositesense,

FIG. 8 shows a method for production of unbalance voltages,

FIG. 9 shows a method for production of a network-frequencynegative-phase-sequence system voltage,

FIG. 10 shows means for simultaneous compensation for vertical andhorizontal unbalances in the opposite sense,

FIG. 11 shows the structure of the regulation means of the apparatusshown in FIG. 1, and

FIG. 12 shows the application of circulating-voltage nominal values toother nominal values of the regulation means.

DESCRIPTION OF THE INVENTION

FIG. 1 shows one exemplary embodiment of the apparatus 1 according tothe invention, comprising three phase modules 2 a, 2 b and 2 c. Eachphase module 2 a, 2 b and 2 c is connected to a positive DC voltage linep and to a negative DC voltage line n, as a result of which each phasemodule 2 a, 2 b, 2 c has two DC voltage connections. Furthermore, arespective AC voltage connection 3 ₁, 3 ₂ and 3 ₃ is provided for eachphase module 2 a, 2 b and 2 c. The AC voltage connections 3 ₁, 3 ₂ and 3₃ are connected via a transformer 4 to a three-phase AC voltage network5. The phase voltages between the phases of the AC voltage network 5 areU1, U2 and U3, with network currents In1, In2 and In3 flowing. The phasecurrent on the AC voltage side of each phase module is annotated I1, I2and I3. The direct current is I_(d). The phase module branches 6 p 1, 6p 2 and 6 p 3 extend between each of the AC voltage connections 3 ₁, 3 ₂or 3 ₃ and the positive DC voltage line p. The phase module branches 6 n1, 6 n 2 and 6 n 3 are formed between each AC voltage connection 3 ₁, 3₂, 3 ₃ and the negative DC voltage line n. Each phase module branch 6 p1, 6 p 2, 6 p 3, 6 n 1, 6 n 2 and 6 n 3 comprises a series circuitformed by submodules, which are not illustrated in detail in FIG. 1, andan inductance, which is annotated L_(kr) in FIG. 1.

FIG. 2 shows a more detailed illustration of the series circuit of thesubmodules 7 and, in particular, the formation of the submodules bymeans of an electrical equivalent circuit, with only the phase modulebranch 6 p 1 having been picked out in FIG. 2. However, the remainingphase module branches are of identical design. As can be seen, eachsubmodule 7 has two series-connected power semiconductors T1 and T2which can be turned off. By way of example, power semiconductors whichcan be turned off are so-called IGBTs, GTOs, IGCTs or the like. Theseare known per se to a person skilled in the art, and there is thereforeno need to describe them in detail at this point. A freewheeling diodeD1, D2 is connected back-to-back in parallel with each powersemiconductor T1, T2 which can be turned off. A capacitor 8 is connectedas an energy storage unit in parallel with the series circuit of thepower semiconductors T1, T2 which can be turned off and the respectivefreewheeling diodes D1 and D2. Each capacitor 8 is charged on a unipolarbasis. Two voltage states can now be produced at the two-pole connectingterminals X1 and X2 of each submodule 7. If, for example, a drive unit 9produces a drive signal which switches the power semiconductor T2 whichcan be turned off to its switched-on position, in which a current canflow via the power semiconductor T2, as a voltage of zero between theterminals X1, X2 of the submodule 7. In this case, the powersemiconductor T1 which can be turned off is in its switched-offposition, in which any current flow via the power semiconductor T1 whichcan be turned off is interrupted. This prevents discharging of thecapacitor 8. If, in contrast, the power semiconductor T1 which can beturned off is in its switched-on position, but the power semiconductorT2 which can be turned off is changed to its switched-off position, thefull capacitor voltage Uc is present between the terminals X1, X2 of thesubmodule 7.

The exemplary embodiment of the apparatus according to the invention asshown in FIGS. 1 and 2 is also referred to as a so-called multi-levelconverter. A multilevel converter such as this is suitable, for example,for driving electrical machines, such as motors or the like.Furthermore, a multilevel converter such as this is also suitable foruse in the field of power distribution and transmission. By way ofexample, the apparatus according to the invention is used as aback-to-back link which comprises two converters connected to oneanother on the DC voltage side, with each of the converters beingconnected to an AC voltage network. Back-to-back links such as these areused to exchange energy between two power distribution networks, forexample with the power distribution networks having a differentfrequency, phase angle, star-point connection or the like. Furthermore,applications may be considered in the field of power factor correction,as so-called FACTS (flexible AC transmission systems). High-voltagedirect-current transmission over long distances is also feasible usingmulti-level converters such as these.

In order to avoid unbalanced distribution of the energy between thesubmodules 7, that is to say between the capacitors 8 of the submodules7, the first action within the scope of the invention is to determinewhether any unbalances are present.

FIG. 3 schematically illustrates a method for detection of any verticalunbalance. For this purpose, the branch energy actual values UcΣp1, . .. , UcΣn3 are first of all determined for each phase module branch 6 p1, . . . , 6 n 3. This is done by measuring the voltage Uc across thecapacitor 8 for each submodule 7. As is shown by the arrow pointing tothe right in FIG. 2, the capacitor voltage value Uc detected by thevoltage sensor is transmitted to the evaluation unit 9. The evaluationunit 9 adds all the capacitor voltage values Uc of a phase module branch6 p 1, . . . , 6 n 3 to form branch energy actual values UcΣp1, . . . ,UcΣn3. In this case, it is irrelevant whether the submodule is connectedto the series circuit and is or is not making any contribution. In orderto obtain a measure for the stored energy, it is also possible to squarethe voltage Uc across the capacitors to form Uc², and then to add Uc² toform the branch energy actual values.

In this case, the branch energy actual values therefore correspond tobranch voltage actual values UcΣp1, . . . , UcΣn3. These are eachconverted by a proportional regulator 10 to form intermediate values,and the intermediate values of the phase module branches 6 p 1, 6 p 2, 6p 3 which are connected to the positive DC voltage connection p areadded to one another. A corresponding procedure is adopted for theintermediate values of the phase module branches 6 n 1, 6 n 2, 6 n 3which are connected to the negative DC voltage connection n. Thisresults in a positive branch sum and a negative branch sum, which aresubtracted from one another by means of the subtractor 11, thus formingthe circulating-current nominal value Dvb in order to compensate for anyvertical unbalance.

FIG. 4 illustrates the production of a network-frequencypositive-phase-sequence system nominal voltage. First of all, both asine function and a cosine function are formed with the argument ωt andthe addition of a phase shift δ. In this case, ω corresponds to thefrequency of the voltage of the connected network. The cosine functionand the sine function are each multiplied by an amplitude which isformed from the circulating-current nominal value Dvb, using aproportional regulator 10. The subsequent conversion of thetwo-dimensional vector space to the three-dimensional space results inthe network-frequency positive-phase-sequence system nominal voltageuvb1, uvb2 and uvb3. These are applied to other nominal voltages in acurrent regulation unit.

On the basis of the circulating-current nominal value Dvb formed asshown in FIG. 3, it is also possible to produce an unbalance voltageUasyn instead of producing a network-frequency positive-phase-sequencesystem nominal voltage. For this purpose, as is shown in FIG. 5, thecirculating-current nominal value Dvb is applied to the input of aregulator 10 which, for example, is a proportional regulator. Theunbalance voltage Uasyn can be tapped off at the output of the regulator10.

FIG. 6 illustrates the verification of a horizontal unbalance in thesame sense. For this purpose, the branch energy actual values UcΣp1, . .. , UcΣn3 of the phase module branches 6 p 1, . . . , 6 n 3 of the samephase module 2 a, 2 b, 2 c are each added to form phase module energysum values, with the branch energy actual values previously having beenamplified by the regulator 10 in proportion to intermediate values. Anadder 12 is used for addition. The averager 13 forms the mean value ofthe phase module energy sum values at the output of the adder 12, andthe subtractor 11 subtracts this from each phase module energy sum valueof one phase. Vertical circulating-current nominal values Dhgl1, Dhgl2,Dhgl3 for each phase can be tapped off at the output of each subtractor11.

FIG. 7 illustrates how a horizontal unbalance in the opposite sense canbe verified. For this purpose, the branch energy actual values UcΣp1, .. . , UcΣn3 are once again first of all amplified by a regulator 10. Incontrast to the method shown in FIG. 6, the difference between thebranch energy actual values UcΣp1, . . . , UcΣn1 of the phase modulebranches of the same phase module 2 a, 2 b, 2 c is then calculated. Themean value is once again formed from the difference over all threephases, with the mean value being subtracted from said difference.Finally, the horizontal unbalance-current nominal value Dhge1, Dhge2 andDhge3 in the opposite sense for each phase can be tapped off at theoutput of the second subtractor 11.

FIG. 8 illustrates how a proportional regulator 10 producescirculating-voltage nominal values uhgl1, uhgl2 and uhgl3 from thecirculating-current nominal values Dghl1, Dghl2, Dhgl3. As alreadydescribed, these circulating-voltage nominal values are fed into theregulation system, thus setting the desired circulating currents tocompensate for the balances.

FIG. 9 shows the production of a network-frequencynegative-phase-sequence system voltage uhge1, uhge2 and uhge3. This isdone starting from the horizontal unbalance-current nominal values inthe opposite sense Dhge1, Dhge2 and Dhge3. Said unbalance-currentnominal values are first of all transformed in the two-dimensionalvector space, and are then amplified proportionally by a regulator 10.The amplified unbalance nominal values are used as the amplitude of acosine function and of a negative sine function with the argument ωt andthe phase shift δ. After transformation to the three-dimensional space,the network-frequency negative-phase-sequence system nominal voltageuhge1, uhge2, uhge3 is obtained for feeding into the current regulationunit and for application to further nominal values in the regulationsystem.

FIG. 10 illustrates means for simultaneous compensation for verticalunbalances and horizontal unbalances in the opposite sense. As describedin conjunction with FIG. 7, branch energy actual values UcΣp1, . . . ,UcΣn3 of the phase module branches 6 p 1, . . . , 6 n 3 of a commonphase module are first of all amplified proportionally by a regulator10, and the difference is then formed in the subtractor 11. Cosinefunctions which depend on the network frequency ω and on the phase δ areformed in parallel with this. The cosine functions, which are formedphase-by-phase, are phase-shifted through

$\frac{2\;\pi}{3}$with respect to one another. The phase-shifted cosine functions aremultiplied by the phase branch module difference that results at theoutput of the subtractor 11, as an amplitude, thus resulting in apositive-phase-sequence system nominal voltage uvb1, uvb2 and Uvb3.

FIG. 11 illustrates the structure of the regulation means. Theregulation means comprise a current regulation unit 10 and drive units 9p 1, 9 p 2, 9 p 3, and 9 n 1, 9 n 2 and 9 n 3. Each of the drive unitsis associated with a respective phase module branch 6 p 1, 6 p 2, 6 p 3,6 n 1, 6 n 2 and 6 n 3. The drive unit 9 p 1, for example, is connectedto each submodule 7 of the phase module branch 6 p 1 and produces thecontrol signals for the power semiconductors T1, T2 which can be turnedoff. A submodule voltage sensor, which is not illustrated in thefigures, is provided in each submodule 7. The submodule voltage sensoris used to detect the capacitor voltage across the capacitor 8, as theenergy storage unit of the submodule 7, with a capacitor voltage valueUc being obtained. The capacitor voltage value Uc is made available tothe respective drive unit, in this case 9 p 1. The drive unit 9 p 1therefore obtains the capacitor voltage values of all the submodules 7of the phase module branch 6 p 1 associated with it, and adds these toobtain a branch energy actual value or in this case branch voltageactual value UcΣp1, which is likewise associated with the phase modulebranch 6 p 1. This branch voltage actual value UcΣp1 is supplied to thecurrent regulation unit 10.

Apart from this, the current regulation unit 10 is connected to variousmeasurement sensors, which are not illustrated in the figures. Forexample, current transformers arranged on the AC voltage side of thephase modules 2 a, 2 b, 2 c are used to produce and supply phase currentmeasured values I1, I2, I3, and current transformers arranged on eachphase module are used to produce and supply phase module branch currentsIzwg, and a current transformer which is arranged in the DC voltagecircuit of the converter is used to provide DC measured values Id.Voltage converters in the AC network provide network voltage measuredvalues U1, U2, U3 and DC voltage converters provide positive DC voltagemeasured values Udp and negative DC voltage measured values Udn, withthe positive DC voltage values Udp corresponding to a DC voltage betweenthe positive DC voltage connection p and ground, and with the negativeDC voltage values Udn correspond to a voltage between the negative DCvoltage connection and ground.

Furthermore, nominal values are supplied to the current regulation unit10. In the exemplary embodiment shown in FIG. 11, an in-phase currentnominal value Ipref and a reactive current nominal value Iqref aresupplied to the regulation unit 10. Furthermore, a DC voltage nominalvalue Udref is applied to the input of the current regulation unit 10.It is also possible to use a DC nominal value Idref for the purposes ofthe invention, instead of a DC voltage nominal value Udref.

The nominal values Ipref, Iqref and Udref as well as said measuredvalues interact with one another using various regulators, with a branchvoltage nominal value Up1ref, Up2ref, Up3ref, Un1ref, Un2ref, Un3refbeing produced for each drive unit 9 p 1, 9 p 2, 9 p 3, 9 n 1, 9 n 2 and9 n 3. Each drive unit 9 produces control signals for the submodules 7associated with it, as a result of which the voltage Up1, Up2, Up3, Un1,Un2 and Un3 across the series circuit of the submodules corresponds asfar as possible to the respective branch voltage nominal value Up1ref,Up2ref, Up3ref, Un1ref, Un2ref, Un3ref.

The current regulation unit 10 uses its input values to form suitablebranch voltage nominal values Up1ref, Up2ref, Up3ref, Un1ref, Un2ref,Un3ref.

FIG. 12 shows that, for example, the branch voltage nominal value Uprefis calculated by linear combination of a network phase voltage nominalvalue Unetz1, a branch voltage intermediate nominal value Uzwgp1, a DCvoltage nominal value Udc, a balance voltage nominal value Uasym and abalancing voltage nominal value Ubalp1. This is done mutuallyindependently for each of the phase module branches 6 p 1, 6 p 2, 6 p 3,6 n 1, 6 n 2, 6 n 3. The circulating currents can be set specifically bythe branch voltage intermediate nominal values Uzwg in conjunction withthe branch inductance settings. The balancing voltage nominal valuesUba1 are also used to compensate for unbalances with regard to theamounts of energy stored in the phase module branches 6 p 1, 6 p 2, 6 p3, 6 n 1, 6 n 2 and 6 n 3.

1. An apparatus for conversion of an electric current, comprising: atleast one DC voltage connection; at least one phase module having an ACvoltage connection and a phase module branch formed between each said DCvoltage connection and each said AC voltage connection, each said phasemodule branch having a series circuit composed of submodules each havingan energy storage unit, at least one power semiconductor, and submodulesensors for detecting energy stored in said energy storage unit andobtaining energy storage unit energy values; and a regulation device forregulation of the apparatus in dependence on the energy storage unitenergy values and predetermined nominal values, said regulation devicehaving an addition unit for addition of the energy storage unit energyvalues resulting in branch energy actual values being obtained and meansfor calculation of circulating-current nominal values in dependence onthe branch energy actual values, said regulation device compensating forunbalances in the branch energy actual values in dependence on thecirculating-current nominal values.
 2. The apparatus according to claim1, wherein said at least one DC voltage connection includes a positivevoltage connection and a negative DC voltage connection; furthercomprising addition means for adding the branch energy actual values ofsaid phase module branches, which are connected to said positive DCvoltage connection, to form a positive branch sum and adding the branchenergy actual values of said phase module branches, which are connectedto said negative DC voltage connection, to form a negative branch sum;and further comprising subtraction means forming a difference betweenthe positive branch sum and the negative branch sum to obtain a verticalcirculating-current nominal value to compensate for any verticalunbalance.
 3. The apparatus according to claim 2, further comprisingmeans for producting a network-frequency positive-phase-sequence systemnominal voltage in dependence on the vertical circulating nominalcurrent to compensate for the vertical unbalance.
 4. The apparatusaccording to claim 2, further comprising means for producing anunbalance voltage in dependence on the circulating-current nominalvalues to compensate for the vertical unbalance.
 5. The apparatusaccording to claim 1, further comprising means for verification of acommonly directed horizontal unbalance outputting a verified commonlydirected horizontal unbalance, with said means producing thecirculating-current nominal values in dependence on the verifiedcommonly directed horizontal unbalance.
 6. The apparatus according toclaim 5, further comprising means for producing circulating-voltagenominal values which are respectively associated with said phase module.7. The apparatus according to claim 1, further comprising means forverification of an oppositely directed horizontal unbalance andoutputting a verified oppositely directed horizontal unbalance, withsaid means producing the circulating-current nominal values independence on the verified oppositely directed horizontal unbalance. 8.The apparatus according to claim 7, further comprising means forproducing a network-frequency negative-phase-sequence system circulatingvoltage in dependence on the circulating-current nominal values.
 9. Theapparatus according to claim 1, further comprising means forsimultaneous compensation for oppositely directed vertical andhorizontal unbalances.
 10. A method for conversion of a current by meansof a converter having at least one DC voltage connection, an AC voltageconnection and phase modules each with a phase module branch beingformed between the DC voltage connection and the AC voltage connection,the phase module branch having a series circuit composed of submoduleseach having an energy storage unit and at least one power semiconductor,which comprises the steps of: detecting an amount of energy stored ineach of the energy storage units resulting in energy storage unit energyvalues being obtained; adding all of the energy storage unit energyvalues of the phase module branch resulting in branch energy actualvalues; determining circulating-current nominal values from the branchenergy actual values; and producing circulating currents in the phasemodules in dependence on the circulating-current nominal values in orderto compensate for unbalances.
 11. The method according to claim 10,which further comprises: adding the branch energy actual values of allthe phase modules which are connected to a positive DC voltageconnection for obtaining a positive total sum; adding the branch energyactual values of all the phase module branches which are connected to anegative DC voltage connection for obtaining a negative total sum; anddetermining a difference between the positive and the negative totalsums for obtaining a vertical circulating-current nominal value.
 12. Themethod according to claim 11, which further comprises producing anetwork-frequency positive-phase-sequence system nominal voltage on abasis of the vertical circulating-current nominal value.
 13. The methodaccording to claim 11, which further comprises producing an unbalancenominal voltage on a basis of the vertical circulating-current nominalvalue by means of a proportional regulator.
 14. The method according toclaim 10, which further comprises adding the branch energy actual valuesof all the phase module branches of a phase module for obtaining phasemodule energy sum values which are associated with a respective phase;calculating a mean value of the phase module energy sum values over allthe phases; and calculating a difference from the mean value and therespective phase module energy sum value for each phase, for obtainingcommonly directed horizontal unbalance-current nominal values.
 15. Themethod according to claim 14, which further comprises formingcirculating-voltage nominal values by means of regulators from thecommonly directed horizontal unbalance-current nominal values, and areincluded as a nominal voltage in a regulation process.
 16. The apparatusaccording to claim 10, which further comprises: subtracting the branchenergy actual values of all the phase module branches of the phasemodule from one another for obtaining phase module energy differencevalues which are associated with a respective phase; calculating a meanvalue of the phase module energy difference values over all the phases;and calculating a difference from the mean value and the respectivephase module energy difference value for each phase for obtainingoppositely directed horizontal unbalance-current nominal values.
 17. Themethod according to claim 16, which further comprises producing anetwork-frequency negative-phase-sequence voltage system from theoppositely directed horizontal unbalance-current nominal values.
 18. Themethod according to claim 10, which further comprises: subtracting thebranch energy actual value of a phase module branch which is connectedto a negative DC voltage connection from the branch energy actual valueof the phase module branch of a same phase module which is connected tothe positive DC voltage connection, with a phase branch moduledifference being obtained; and using the phase branch module differenceas an amplitude of a periodic function which oscillates at a networkfrequency and is associated with a phase module, and with periodicfunctions of the other phase modules each being phase-shifted so as toform a positive-phase-sequence system nominal voltage.